In recent years, aquaculture (cultivation of fish or other aquatic organisms) is characterized by a tendency towards growing more fish per unit area. Special ponds have been constructed (Zohar and Rappaport, Zohar, G., Rappaport, U. and S. Sarig, 1985. Intensive culture of tilapia in concrete tanks. Bamidgeh 37: 103-112, 1985; van Rijn et al., van Rijn, J., Stutz, S, Diab, S. and M. Shilo. 1986. Chemical, physical and biological parameters of superintensive concrete fish ponds. Bamidgeh 38: 35-43. van Rijn, J. and G. Rivera. 1990. Aerobic and anaerobic biofiltration in an aquaculture unit--Nitrite accumulation as a result of nitrification and denitrification. Aquacult. Engineer. 9: 217-234. 1986) that enable stocking densities of up to 50 times the stocking densities generally maintained in conventional fishponds. As opposed to conventional fishponds, waterquality deterioration proceeds rapidly in these intensive fishponds and without man-made interference, fish mortality would be imminent. DE-A-38 27 716 describes a system for water quality control in intensive fish culture systems for reducing inorganic nitrogen and organic matter concentrations in an open system. Theoretically, two options exist as to maintaining an adequate waterquality in these intensive fish culture ponds. Either the ponds are continuously flushed with clean (unpolluted) water or the pondwater is continuously treated in order to reduce the level of pollutants. Unlimited amounts of clean water to flush the ponds is a luxury which is restricted to a few geographical areas only. Therefore, treatment of the pondwater is the option of choice in most places.
The accumulation of inorganic nitrogen in intensively cultured fishponds is one of the major limiting factors preventing a further intensification. Inorganic nitrogen (especially ammonia and nitrite) is toxic to fish and it accumulates in the pondwater through excretion of ammonia by the fish and by breakdown of organic solids. Most of the treatment systems used in aquaculture facilities are designed to facilitate the growth of nitrifying bacteria which oxidize ammonia via nitrite to nitrate. A drawback of the ammonia removal by means of nitrification is the subsequent increase in nitrate in the culture system. Nitrate concentrations of up to 800 mg liter.sup.-1 NO.sub.3-- N have been recorded in semi-closed aquaculture facilities where nitrification was employed as water purification step. High nitrate concentrations ought to be prevented for several reasons. Firstly, nitrate at high concentrations has a toxic effect on several fish species. Secondly, the discharge of nitrate-rich effluent water is prohibited in many countries due to environmental and public health considerations. The maximum levels of nitrate allowed in the effluent water differ from country to country and are as low as 11.6 mg liter.sup.-1 NO.sub.3-- N in Europe according to the European Community directive. Thirdly, under certain conditions nitrate in the fish culture system is converted to nitrite, a compound extremely toxic to fish.
A treatment system was developed by our group (van Rijn and Rivera, 1990) which was aimed a reducing inorganic nitrogen from the pond water by means of induction of two microbial process: nitrification (oxidation of ammonia to nitrate) and denitrification (reduction of nitrate to N.sub.2 gas). Nitrification was induced in a socalled trickling filter containing material previously not used for this purpose. Denitrification was induced in fluidized bed reactors. As denitrification is a heterotrophic process (denitrifying bacteria require organic matter for growth and metabolism), organic matter derived from the fishpond was led through the fluidized bed reactor. This treatment system was not entirely satisfactory as denitrifying activity in the fluidized bed reactors was highly unpredictable and large daily fluctuations in nitrate removal were observed. Furthermore, nitrite accumulation by the fluidized bed reactor was found under all running conditions tested (van Rijn and Rivera, 1990).